The invention relates to densification of porous bodies. One particular instance is in the densification of porous structural components such as brake discs or pads for aircraft, which typically are made of a porous carbon body infiltrated by carbon.
Many different methods are known for the deposition of a carbon or ceramic matrix on a fibrous substrate of carbon or ceramic fibres. In a thermal gradient process the densification is started at the inside of the substrate or preform and progresses outwards until the whole article is densified. Because the exterior pores of the preform do not become blocked until the centre of the preform is densified, it is found that densification can proceed at a higher rate, reducing processing times and cost compared to the isothermal method of densification.
In all processes where a fibrous substrate is being densified the control of deposition conditions, such as temperature and pressure, is important to ensure the deposition reaction continues at the required rate in order to produce a deposit of the required structure. Where a matrix of carbon is being deposited to manufacture friction discs for aircraft brakes, it can be desirable that the deposit is a graphitisable carbon in order to heat treat the composite to maximise the thermal properties. (The graphitisable form of carbon is often referred to as rough laminar and can be distinguished from the non-graphitisable smooth laminar form by microscopic examination using polarised light).
In thermal gradient deposition heating is commonly by induction heating of a graphite susceptor see, e.g. U.S. Pat. No. 5,348,774 which discloses a process in which the fibrous substrate is in contact with a graphite mandrel which is heated by the electromagnetic field from an induction coil. The fibrous substrate is then heated by conduction from the graphite mandrel. The main problem with this method of establishing the thermal gradient in the preform is the requirement for a susceptor core which might only be usable once if damage is caused during removal from the densified composite.
Another method of producing a thermal gradient in a fibrous substrate for densification is by chemical vapour infiltration (CVI) and is described by J. J. Gebhardt et al in a paper entitled xe2x80x9cFormation of carbon-carbon composite materials by pyrolitic infiltrationxe2x80x9d, Petroleum Derived Carbons ACS Ser. No. 21 6/73, pages 212 to 217. In the method of Gebhardt the thermal gradient is maintained by a high rate of gas flow cooling the outer surface of the substrate and by heating only a small volume of the cylindrical substrate in the induction coil, densification of the whole being achieved by moving the substrate inside the induction coil. The substrate is manufactured from graphite fibres which have sufficient electrical conductivity to enable heating of the substrate by direct coupling with the induction coil. Practical constraints on the process make it difficult to apply on an industrial scale and it is unsuitable for other substrate geometry, such as a disc.
U.S. Pat. No. 5,390,152 discloses a method of densification in one version of which (FIG. 6), a porous body is placed on a support between so-called horizontal coils or pancake coils. It is suggested that the spacing between the turns of the coil may be non-uniform in some instances. It is also suggested that as the preform porous body begins to densify the frequency of the applied current may have to be increased to compensate for a decrease in resistivity or the current may need to be increased for decreased resistance.
It is one object of the invention to provide an improved method of densification in which the heating is controlled to improve the rate of deposition after graphitisation.
In one aspect there is provided a method of infiltrating a porous preform body with carbon by heating the body having different portions to a temperature at which carbon-containing gas is crackable to deposit carbon in the pores thereof, the method comprising placing the body adjacent a multi-portion heating element so that the portions thereof align with corresponding portions of the body, and supplying power to the heating element to heat the body whilst exposing the body to a crackable carbon-containing gas; characterised by selectively energising different portions of the multi-portion heating element to create a thermal gradient between one heated body portion and the or each adjacent body portion so as to selectively deposit carbon in the pores of each different heated body portion to provide a graphitisable rough laminar structure.
Preferably adjacent portions of the multi-portion heating element are used to heat adjacent portions of the body to different temperatures whereby to create a thermal gradient between the respective portions of the body.
DE-A-4142261 discloses a method of infiltrating a porous body with a gas stream by means of a heating element the shape of which can be matched to that of the body. The document teaches that for a body having an irregular shape the heating element may be made of plates having a corresponding shape and that such plates may have independent power supplies. There is no suggestion of the creation of a thermal gradient for infiltration.
Preferably the heating element is generally circular. In one variation the heating element comprises a base comprising an induction heating tube shaped into substantially concentric coils, and means are present to power each coil individually. Means may be present to power some coils in combination. In another variation the heating element is made up of vertically aligned coils, arranged to heat layers within the preform.
The porous body will typically be a fibrous preform, of any type. The fibres will be carbon or other suitable fibre.
The infiltrating gas will be one which may be infiltrated by a chemical vapour infiltration technique. The gas may conveniently be provided by a gas vapour or a liquid precursor.
In another aspect the invention provides a method of method of infiltrating a porous preform body with carbon by heating the body having different portions to a temperature at which carbon-containing gas is crackable to deposit carbon in the pores thereof, the method comprising placing the body adjacent a multi-portion heating element so that the portions thereof align with corresponding portions of the body, and supplying power to the heating element to heat the body whilst exposing the body to a crackable carbon-containing gas; characterised by selectively energising different portions of the multi-portion heating element to create a thermal gradient between one heated body portion and the or each adjacent body portion so as to selectively deposit carbon in the pores of each different heated body portion.
Preferably the infiltration of the selected portion is continued until a predetermined level has been achieved following which the process is repeated to adjacent selected portions in succession whereby to infiltrate substantially all of the body to substantially uniform level in a relatively short time period.